Cogeneration refers to the generation of both electricity and useful heat. In the prior art, it has been known to employ a thermodynamic cycle, including the particular type known as a closed loop Organic Rankine Cycle (ORC) to carry out cogeneration using energy provided by the combustion of a fuel.
In a closed loop Rankine cycle, a working fluid follows a closed loop path through which it is continuously recycled. In an Organic Rankine Cycle, the working fluid is an organic fluid such as n-pentane or toluene. An Organic Rankine Cycle may be described as a recurring cycle of four constituent processes. In one process, working fluid is pumped from a low pressure to a high-pressure. In a second process, the liquid working fluid is heated to become a vapor at substantially constant pressure. In a third process, the vapor is expanded through a turbine which is coupled to a generator to generate power while concomitantly decreasing its temperature and pressure. In a fourth process, the vapor is condensed to become a liquid.
For example U.S. Pat. No. 5,704,209 to Bronicke et al. discloses an externally fired combined cycle gas turbine system having a compressor for compressing ambient air, a water heat exchanger for heating the compressed air, an air turbine for expanding the heated compressed air in the generator connected to the turbine for generating electricity. The system also includes a source of energy such as solar energy, oil shale, solid waste fuel, landfill gas, biomass or combinations thereof and/or hydrocarbon fuels for adding heat to the compressed air in the air heat exchanger and producing heat depleted gases exiting the heat exchanger. The system further includes a closed Rankine cycle power plant having a water heat exchanger for vaporizing water and producing steam using heat contained in the heat depleted gases and steam turbine for expanding the steam and producing power and expanded steam.
U.S. Pat. No. 7,882,692 to Pronske et al. discloses a fuel combustion power generation system operating as a closed Rankine cycle with a working fluid that is externally heated by combustion of fuel in the presence of oxygen in a combustor. Products of the combustion, typically including steam and CO2, are routed through a high temperature side of a heat exchanger through whose low temperature side passes the working fluid of the closed Rankine cycle. The working fluid is passed on to a turbine, or other expander, which in turn is coupled to a generator for a.c. electrical power generation. The fuel may be gaseous fuel or a solid or liquid fuel, such as coal or biomass, but gasification before combustion is disclosed.
Maxxtec AG of Sinsheim, Germany offers organic Rankine cycle modules in which combustion of biomass produces combustion gases at temperatures of approximately 950° C. which, via a heat exchanger, heats thermal oil to a temperature of 300° C. to 330° C. A secondary circuit having a second heat exchanger uses the thermal oil to heat organic fluid for the organic Rankine cycle. The heated organic fluid is evaporated in a turbine and drives the turbine to generate electricity by way of a generator to which the turbine is mechanically coupled. Water cooled condenser(s) condense the organic vapor and heated cooling water is available for use for various heating applications such as heating dryers or hot water heating systems. Although solid wood biomass may be used as fuel, such must be chipped, shredded or otherwise fractionalized into small, substantially uniform pieces prior to combustion.